The question preoccupying economic policymakers is not the depth but the shape of the recession. The global economy is caught in a downturn as steep as the Great Depression. But in slashing interest rates, rescuing the banking system and pumping money into the economy, governments and central banks hope that recovery will be swift and sharp. And because economists typically prefer images to words, they describe that recovery as “V-shaped”: the downward stroke of the letter represents the collapse in growth, and the upward stroke the corresponding recovery beginning perhaps in the middle of this year.

That is the most hopeful case for the global economy. But there are many risks and different possible scenarios. Robert Shiller, a Yale economist, spoke in London this week with a more cautious message. He suggested that the strong recent stock market performance might be merely a burst of false confidence at the start of a decade of recession in the UK.

This would be not a V-shape, but an L-shaped trajectory for the economy, in which consumption and investment resisted all attempts to stimulate growth. And having foreseen the bursting of the dot-com bubble and the collapse of the housing market, Professor Shiller has a good claim to understand where structural stresses in the economy lie.

Vs and Ls do not exhaust the alphabetical metaphors. Professor Shiller's downbeat assessment would be consistent also with a U-shaped recovery, in which the damage to the economy prevents a strong recovery. Then there is a W shape, in which policymakers, prematurely impressed with the effectiveness of their efforts, tighten monetary and fiscal policy too quickly and thereby plunge the economy back into recession. One imaginative economist has recently suggested a “square-root shaped” recovery, corresponding to the mathematical sign. On this scenario, the recovery is initially sharp but then levels off.

If the human costs of the recession were not so obvious, it might seem faintly comic to be wondering which letter of the alphabet or symbol the economy most resembled. But there is a serious purpose behind it. Economic growth depends on confidence. John Maynard Keynes explained in the 1930s that businesses would invest only if they expected demand for their goods. If confidence were absent, then the economy might merely stagnate, regardless of how low interest rates fell. This is what happened in Japan in the “lost decade” of the 1990s, when no amount of monetary and fiscal easing could stimulate growth.

Keynes coined, and Professor Shiller has recently revived, the phrase “animal spirits” to signify this ingredient in economic recovery. These are not merely an expression of confidence in the future. They are more a shared sense of trust. The depth of the recession testifies to the waning of trust. In the boom, banks assumed that complex financial instruments had reduced the risks of lending. In reality, sub-prime mortgages and other asset-backed securities contaminated the financial system and brought down the real economy. Banks then hoarded cash rather than lent it, for fear of not getting it back.

Policy for the recession must therefore do more than expand the money supply and boost spending. It needs to give people a sense that it is rational to trust - to trust, for example, that banks will not be allowed to fail and that savings are safe. Policymakers must anticipate the ways that recovery might fall short, and convince the public that all is being done to restore growth.

Economic policy is not as easy as ABC. But being clear on the urgency of V might help to avoid L, U and W.